1. Field of the Invention
The invention is related to methods and apparatus for controlling the fluid level in an open pit, particularly for use in controlling the brine level in a gravity-fed batch process used in water treatment plants.
2. Description of Related Art
Devices, which have previously been used for controlling the fluid level in a pit, or sump, include bubbler tubes or xe2x80x9cbubblersxe2x80x9d. In a bubbler, instrument air is used to purge the measuring tube whereby the air forms bubbles that rise through the measured fluid. The head pressure is proportional to the height of the fluid level. In this system, the tube becomes bridged with solids (salts) to such an extent that it will no longer bubble and, therefore, requires shutting down the system for maintenance. Failure of the bubbler (or any other level control system) in a brine pit leads to a lack of brine for regeneration and to eventual pump failure. One prior art level transmitter used a bubbler tube that was constantly plugging due to salt buildup at the end where the tube contacted the process. The bubbler system was redesigned to utilize an electronic transmitter with a 2-inch diameter bubbler tube having multi-ported piping. Having multiple ports helped the fluid to exit the piping section even when one port plugged up. However, many ports became plugged and this design also failed. A Programmable Logic Controller (PLC) was used to backflush the bubbler tube with air solenoids, however, the bubbler tube input continued to plug with crystallized salt despite the use of both instrument air and water purges.
The present invention is a 0-100% continuously adjustable, low deadband, point contact pneumatic level switch for use in controlling the fluid level in a pit, particularly in a brine pit. It is used in conjunction with an electronic controller, a pneumatic-to-current (P/I) converter and a current-to-pneumatic (I/P) converter to control brine level in a gravity-fed batch process. The invention comprises three elements: (1) a float ball system for tracking the fluid level in a sump; (2) a pneumatic level switch which includes a nozzle/flapper assembly for signaling when the fluid level is at a selected level or deviates therefrom; and (3) a regulator for supplying instrument air to the pneumatic level switch and to the P/I converter for use in controlling the fluid level in the pit. The invention eliminates the prior art air purge bubbler problem by employing a float that controls a nozzle/flapper assembly which, in turn, controls the flow of make-up water into the sump. The nozzle/flapper assembly does not contact the brine fluid and therefore does not clog the system. It allows very close tolerance control of the fluid level in an open pit at any point on a scale ranging from 0% (empty) to 100% (full). The level switch has an extremely low deadband of {fraction (1/32)} inch of the open pit height of seventeen feet. As brine solution is demanded, the pit fluid level drops, at which time the switch sends a signal to a controller to open a water valve to make up a new batch of brine and to reset the fluid level to the 60% set point. When the fluid level reaches the selected set point setting, the switch sends a signal to the controller to close a water valve and shut off water to the sump. The invention also eliminates the level measurement tracking error i.e., failure of an instrument to follow the changes in actual fluid level of an open pit.